In order to avoid having userspace to use MI_MEM_FENCE,
we are adding a mechanism for userspace to generate a
PCI memory barrier with low overhead (avoiding IOCTL call
as well as writing to VRAM will adds some overhead).
This is implemented by memory-mapping a page as uncached
that is backed by MMIO on the dGPU and thus allowing userspace
to do memory write to the page without invoking an IOCTL.
We are selecting the MMIO so that it is not accessible from
the PCI bus so that the MMIO writes themselves are ignored,
but the PCI memory barrier will still take action as the MMIO
filtering will happen after the memory barrier effect.
When we detect special defined offset in mmap(), We are mapping
4K page which contains the last of page of doorbell MMIO range
to userspace for same purpose.
For user to query special offset we are adding special flag in
mmap_offset ioctl which needs to be passed as follows,
struct drm_xe_gem_mmap_offset mmo = {
.handle = 0, /* this must be 0 */
.flags = DRM_XE_MMAP_OFFSET_FLAG_PCI_BARRIER,
};
igt_ioctl(fd, DRM_IOCTL_XE_GEM_MMAP_OFFSET, &mmo);
map = mmap(NULL, size, PROT_WRITE, MAP_SHARED, fd, mmo);
IGT : b2dbc6f228
UMD : https://github.com/intel/compute-runtime/pull/772
V7:
- Dgpu filter added
V6(MAuld)
- Move physical mmap to fault handler
- Modify kernel-doc and attach UMD PR when ready
V5(MAuld)
- Return invalid early in case of non 4K PAGE_SIZE
- Format kernel-doc and add note for 4K PAGE_SIZE HW limit
V4(MAuld)
- Add kernel-doc for uapi change
- Restrict page size to 4K
V3(MAuld)
- Remove offset defination from UAPI to be able to change later
- Edit commit message for special flag addition
V2(MAuld)
- Add fault handler with dummy page to handle unplug device
- Add Build check for special offset to be below normal start page
- Test d3hot, mapping seems to be valid in d3hot as well
- Add more info to commit message
Cc: Matthew Auld <matthew.auld@intel.com>
Acked-by: Michal Mrozek <michal.mrozek@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250113114201.3178806-1-tejas.upadhyay@intel.com
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